An understanding of microtubule assembly/disassembly dynamics is sought that would explain spindle formation and certain aspects of chromosome movement. Earlier studies of experimentally controlled chromosome distribution suggested possible molecular mechanisms that would be tested by studies of chromosomes detached from the spindle by micromanipulation, as well as by descriptive studies on the earliest stages of spindle formation in normal cells. The regulation of chromosome movement would be investigated in experiments in which a force of known relative magnitude is applied to a chromosome in mitosis.